A WDM (Wavelength Division Multiplexing) system multiplexing optical signals having different wavelengths is utilized as an optical transmission system. In addition, in a 10-Gbps optical transmission system, a NRZ (Non Return Zero) modulation system that modulates the optical intensity is utilized. In addition, in order to achieve the high-speed transmission, in 40-Gbps and 100-Gbps optical transmission systems, phase modulation systems such as a QPSK (Quadrature Phase Shift Key) modulation system and a DQPSK (Differential Quadrature Phase Shift Key) modulation system are considered.
An optical transmission system is formed from relay stations. The transmission of a desired optical signal becomes possible by arranging a regenerative repeater station capable of regenerating an optical signal in some of relay stations. Japanese Laid-Open Patent Publication No. 2004-48477 discloses a method of arranging a regenerative repeater station in an optical transmission system including relay stations.